Can confirm what APG says about Pak or Bok Choi.. dont bother waiting till it gets warm for sewing. It will go to seed. Much better to grow either side of the summer. Early Spring or Autumn is best. I am still growing Pak Choi in my Poly Tunnel along with spring onions and kale. This was what I collected on Boxing Day, New potatoes, Spring onions, Beetroot, and Pak Choi.. a fresh winter salad !
https://ibb.co/h1cL6FM
Fennel is another one not to forget that prefers the colder months.